The basic concept of cellular communication system involves the principle that a mobile terminal should be able to change base station, preferably without disturbing any ongoing traffic. The mobile thus has the need to be able to find different base stations to communicate with. Since the mobile terminals may move around, there has to be a continuous, or at least frequently intermittent, search for the existence of other base stations within communication distance. In case the signal strength of the base station with which the mobile terminal currently communicates is weakened significantly, it might be beneficial to perform a handover to another base station with higher signal strength or better signal-to-noise ratio.
In a communication system that uses Orthogonal Frequency Division Multiplexing (OFDM), the data is sent in parallel over several carriers. Since the data is transmitted in parallel, the time duration of each OFDM symbol can be made much longer than the length of the time dispersion of the radio channel. To further increase the robustness a cyclic extension, or a Cyclic Prefix (CP), is usually inserted. As long as the length of the CP is longer than the length of the channel impulse response there will be no Inter-Symbol Interference (ISI) in the OFDM system.
In order for the mobile to be able to receive data from a base station using OFDM for downlink transmission, the mobile must be time-synchronised to the base station. The mobile needs to know the beginning of the useful symbol duration. In a cellular system with unsynchronised base stations, the timing of different base stations may drift and a mobile can not easily detect any data sent from another base station with a different time synchronisation.
In order to initiate a handover, the mobile needs to detect the existence of at least a second base station within communication distance. The mobile also needs to know some identity number, or cell ID, of this second base station. If the mobile is unsynchronised to the second base station, then the mobile has to search for signals that might originate from a second base station. The mobile has to assume a preliminary time synchronisation and determine if there is any meaningful information available when using such a time synchronisation. In case the information is un-interpretable, the preliminary time synchronisation has to be changed and a new evaluation of any meaningful information is performed. Such matching between expected and received signal patterns is performed for a number of possible base stations. This procedure is demanding in terms of processing power, and thereby also in terms of battery resources of the mobile station. Furthermore, in a typical case, such a process will also take a considerable time, during which the call may be lost due to the weak present connection.